1. Field of The Invention
This invention relates to an optical fiber light coupling interface between a light source, such a laser diode or LED and the end of an optical fiber and the method for making same.
2. Prior Art
A number of proposals have been made for an optical fiber interface wherein a graded index lens (i.e., Selfoc lens) and/or a convex lens is inserted into the gap between a light source and an optical fiber. This type of optical fiber interface has such problems that the optical axes of the respective elements can not easily be aligned, and that the inclination of the optical axis of a certain element, even if the inclination is a minor one, drastically increases the light coupling loss. These problems prompted a number of studies on the direct coupling of light power from a light source to the optical fiber. For instance, a direct light coupling system has been described in a paper entitled "A new scheme of coupling from LD to SMF utilizing a beam-expanding fiber with a spherical end", Shirai, et.al., presented at the 1990 Spring National Conference of the Institute of Electronics, Information and Communication Engineers.
A basic configuration of a light coupling device to couple the light source to the single-mode optical fiber, wherein the light source, i.e., a laser diode (LD) or a light emitting diode (LED), is directly coupled to the optical fiber end-face without the use of any optical lens systems, will be described hereafter.
FIG. 6 shows an example of the above mentioned light coupling device to couple the light source to the single-mode optical fiber, wherein the light power input end-face is perpendicular to the optical axis thereof. A light power 4 which radiates from a light source 1, i.e., a laser diode (LD), enters into an optical fiber end-face consisting of an optical fiber core 2 and an optical fiber clad 3.
The light power 4 which is radiated from the laser diode (LD), if a laser diode is used as the light source, is produced by a coherent beam distributed around the optical axis thereof in accordance with the Gaussian function in terms of a radiation angle .theta..sub.1 of the light power 4 and will cause diffraction. The radiation pattern is thus elliptical in shape. The elliptically shaped light beam has a major axis XX' which measures 40 to 60 degrees from the light source and a minor axis YY' which measures 20 to 30 degrees from the light source.
Transmission angle .theta..sub.2 at which the light power can be transmitted within optical fiber core 2 should be equal to or less than threshold angle .theta..sub.c. That is: EQU .theta..sub.2 .ltoreq..theta..sub.c =cos.sup.-1 (n.sub.2 /n.sub.1)
where n.sub.1 is the refractive index of the optical fiber core, and n.sub.2 is the refractive index of the optical fiber clad. If we assume n.sub.1 =1.47 and n.sub.2 =1.467 for a single-mode optical fiber, then .theta..sub.c =3.661 degrees can be obtained. PA1 .theta..sub.c : threshold angle of the optical fiber core. PA1 .theta..sub.2 : transmission angle of the light beam entered into the optical fiber core. PA1 .theta..sub.1 : radiation angle of the light beam from the light source. PA1 n.sub.1 : refractive index of the optical fiber core. PA1 inserting an optical fiber into a ferrule and fastening the optical fiber to the ferrule; PA1 forming a conical surface at the end surface of the optical fiber by using a cylindrical grinder; and PA1 forming a spherical surface at the end of the conical surface of the optical fiber by using a spherical grinder.
If transmission angle .theta..sub.2 satisfies the expression .theta..sub.2 .ltoreq..theta..sub.c for the transmission of the light power within the optical fiber core 2, the value n.sub.0 sin .theta..sub.1 should be equal to or less than the numberical aperture NA of the optical fiber core. That is: EQU n.sub.0 sin .theta..sub.1 .ltoreq.NA=n.sub.1 sin .theta..sub.c
where n.sub.0 is the refractive index of air (n.sub.0 =1).
The numerical aperture NA of the optical fiber core for a threshold angle .theta..sub.c of 3.661 degrees is easily determined since the values of n.sub.1 and .theta..sub.c are nown. Thus, .theta..sub.c is calculated as being less than or equal to 5.38 degrees (.theta..sub.1 .ltoreq.5.38). If the average radiation angle for all light power flux 4 is 25 degrees, and the corresponding effective indicent angle .theta..sub.1 to the end-face of optical fiber core 2 is limited to 5.38 degrees or less (.theta..sub.1 .ltoreq.5.38), the percentage of the light power transmitted through the optical fiber core for a single-mode optical fiber to that which is radiated from the light source is approximately 20% if all other losses are disregarded.
If the light power enters on the optical fiber core at an incident angle .theta..sub.1 of greater than 5.38 degrees, it also enters into the optical fiber clad 3, and will therefore be lost from the optical fiber core during the transmission through the optical fiber. This is the reason that the light coupling loss occurs. As suggested in FIG. 6, transmission angle .theta..sub.2 increases with radiation angle .theta..sub.1 and finally becomes greater than threshold angle .theta..sub.c causing light coupling loss. In order to increase the light coupling efficiency measured from the light source to the optical fiber core, transmission angle .theta..sub.2 should be equal to or less than threshold angle .theta..sub.c for all radiation angles .theta..sub.1. A direct light coupling system of this type has thus not been used in most light coupling.
As described heretofore, the light coupling efficiency at the interface between the light source and the optical fiber in the direct light coupling system is inherently low. Many experiments have been done to improve the light coupling efficiency in the direct light coupling system. Among those, a typical example is shown in FIG. 5.
An optical fiber consisting of an optical fiber core 5 and an optical fiber clad 6 is fused, drawn, and cut to form tapered section 7 whose diameter is gradually decreased toward the end-face thereof so that mode radius .omega. of the optical fiber core 5 is extended toward the end-face thereof. A hemispherically shaped microlens 8 is formed at the end of the optical fiber so as to improve the light coupling efficiency.
An example of the above optical fiber is described in IEEE Journal of Lightwave Technology, Vol. 11, No. 2, pp. 252-257 (February 1993).
If the ratio of distance S between the light source and the optical fiber end-face to radius R of microlens 8 is properly defined in the aforementioned example, transmission angle .theta..sub.2 can be equal to or less than threshold angle .theta..sub.c in a wide range of radiation angles .theta..sub.1. In this case, incident angle .alpha. is equal to (.theta..sub.1 +.theta..sub.a) where .theta..sub.a will be described hereafter. If incident angle .alpha. is greater than Brewster's angle .theta..sub.B, the reflection of the light power at the optical fiber end-face increases and the transmitted light power decreases.
Brewster's angle .theta..sub.B is expressed as follows: EQU .theta..sub.B =tan.sup.-1 n.sub.2
.theta..sub.B is 55.77 degrees for n.sub.2 =1.47. As radiation angle .theta..sub.1 increases, angle .theta..sub.a between point P.sub.1 on the hemisphere and optical axis ZZ' rapidly increases. That is; incident angle .alpha. becomes equal to Brewster's angle .theta..sub.B for a small amount of increase in radiation angle .theta..sub.1. According to calculations, incident angle a is nearly equal to .theta..sub.B (.alpha..apprch..theta..sub.B) at radiation angle (.theta..sub.1) of approximately 18 degrees for S=1.6R. Transmission angle .theta..sub.2 is nearly equal to 3.6 degrees (.theta..sub.2 .apprch.3.6.degree.) at a radiation angle of approximately 18 degrees, which is nearly equal to optical fiber threshold angle .theta..sub.c. Note that this fact is important.
The light power radiated at a radiation angle of greater than 18 degrees (.theta..sub.1 &gt;18.degree.) is not propagated along the optical fiber core causing the light power loss. According to the IEEE Journal of Lightwave Technology, Vol. 11, No. 2, pp. 252-257 (February 1993), a paraboloidal surface of an optical fiber end-face (not shown), which is formed by a laser welding machine, is proposed so that transmission angle .theta..sub.2 is set to be nearly equal to 0 degree (.theta..sub.2 .apprch.0.degree.) regardless of radiation angle .theta..sub.1. The optical property change due to fusing of the optical fiber end-face, however, is unknown in this proposal, which leaves room for improvement in processing machines and techniques.
A paper entitled "A New Scheme of Coupling from LD to SMF Utilizing a Beam-expanding Fiber with a Spherical End", by Shirai et. al., presented at the 1990 Spring National Conference of the Institute of Electronics, Information and Communication Engineers, described an optical fiber having a spherical end-face, and points out that the optical loss due to inclination of the optical axis for the spherical end-face is Greater than that for the standard sinGle-mode optical fiber end-face. See FIG. 6 for details.
As described heretofore, a light coupling lens wherein an optical fiber end-face structure such as that which is disclosed in this specification is formed at the front end of the light coupling lens. This lens has been disclosed in Japanese Patent Application KOKAI 1987-81615 in 1987. Obtaining optical axis alignment with the disclosed structure is difficult since the inclination of the optical axis greatly affects the light coupling loss, which has been described heretofore.
As described heretofore, effective light coupling from the light source directly to the optical fiber is difficult; however, direct light coupling from the light source to the optical fiber is attractive in that it allows the designer to construct a light coupling device of simple structure.